1. Field of the Invention
The present invention relates generally to electrical energy storage capacitors, and more particularly to energy storage capacitors suitable for use in an implantable cardiac defibrillator.
2. Background Information
Implantable defibrillators are implanted in patients who are at risk of suffering cardiac arrhythmias, such as ventricular fibrillation, that can cause sudden death. The defibrillator detects the occurrence of ventricular fibrillation and automatically delivers defibrillating therapy in the form of a high-energy shock to the cardiac tissue. Implantable defibrillators in their most general form include appropriate electrical leads and electrodes for collecting electrical signals generated by the heart, and for delivering electric shocks to the heart. Also included are batteries and energy storage capacitors, and control circuitry connected to the leads, batteries and capacitors. The control circuitry senses the electrical activity of the heart and controls the charging of the capacitors and the delivery of the shocks through the leads to the heart.
Defibrillation therapy generally involves rapid delivery of a relatively large amount of electrical energy to the heart at high voltage. Typical values include 20 joules or more at 700 volts or more. Presently available batteries suitable for use in implantable defibrillators are not capable of delivering energy at such levels directly. Consequently, it is customary to provide a high-voltage energy storage capacitor that is charged by the battery via appropriate voltage transformation and charging circuitry. To avoid wasting battery energy, the high-voltage energy storage capacitor is not maintained in a charged state, but rather is charged after fibrillation has been identified by the control circuitry and immediately prior to delivering the shock.
The amount of electrical energy that must be transferred to cardiac tissue to effect defibrillation is quite large by the standards of other implantable cardiac stimulators, such as pacemakers and cardioverters, which treat bradycardia and tachycardia, respectively. Consequently, the physical volume of the energy storage capacitors employed in implantable defibrillators is substantial. Together with the battery, the energy storage capacitor presents a major limitation to reducing the overall size of the implanted device.
Conventional energy storage capacitors used in implantable defibrillators have employed an aluminum electrolytic capacitor technology that had been developed for photoflash capacitors. Aluminum electrolytic capacitors have plates of aluminum foil separated by a porous layer, often paper, impregnated with a viscous liquid electrolyte comprising ethylene glycol plus additives. Alternating layers of foil and paper are wound in a spiral about a mandrel to form a cylindrical capacitor. Electrical leads are attached to respective separate foil layers. The wound capacitor is placed in a cylindrical aluminum can, or housing, closed at one end and open at the other. The dielectric is formed at the electrolyte-to-plate interface by applying a controlled direct current between the leads of the capacitor. Periodically throughout life of the capacitor, especially after periods of non-use, that same process must be used to re-form the dielectric of the aluminum electrolytic capacitor. To complete the construction of the aluminum electrolytic capacitor, the open end of the aluminum can is closed by an elastomeric seal, through which the electrical leads project. The elastomeric seal prevents leakage of electrolyte from the aluminum can, but does not provide an hermetic seal. This permits venting of hydrogen gas that is normally liberated in the aluminum electrolytic capacitor during use.
While aluminum electrolytic capacitors have been used successfully in implantable defibrillators, certain of their characteristics are regarded as disadvantageous. For example, the outgassing characteristic is undesirable in a capacitor that is contained within an implantable device that itself must be hermetically sealed against intrusion by body fluids. The device either must be provided with internal hydrogen adsorbers or else made permeable to hydrogen to prevent an internal buildup of pressure. The relative thickness of the aluminum foil plates and paper separators, as well as the head room required at the ends of the capacitor housing, place upper limits on the energy density of the aluminum electrolytic capacitor, resulting in a relatively bulky device. This is undesirable in the context of pectorally implanted defibrillators which, for reasons related to ease of implantation, comfort and cosmetics, are desired to be as small as possible. Typical aluminum electrolytic photoflash capacitors have energy densities of about 1.8 Joules per cubic centimeter. Also, aluminum electrolytic capacitors typically have a maximum working voltage of about 380 V, whereas implantable defibrillators are usually designed to deliver a shock at 700 V or more. Consequently, two capacitors must be employed in series to achieve the desired working voltage. This results in inefficient space utilization in the implantable device. The need to periodically reform the dielectric of the aluminum electrolytic capacitor is also an undesirable characteristic of a capacitor enclosed in a self contained, battery powered, implanted device. The periodic reformation consumes energy from the battery that otherwise would be available for therapeutic use, thereby reducing the longevity of the implanted device.
Another capacitor technology that has been considered for use in implantable defibrillators is the ceramic dielectric capacitor. The ceramic capacitor has advantages over the aluminum electrolytic capacitor in that it is free of outgassing and does not need periodic reformation. Nevertheless, the ceramic capacitor has been difficult to manufacture with the working voltage and reliability characteristics needed for use in an implantable defibrillator. For example, working voltages above about 400 V have been difficult to achieve. A single local defect in the ceramic dielectric can result in a short circuit between the plates, resulting in catastrophic failure of the capacitor. Also, ceramic capacitors are relatively heavy. Excess weight is undesirable in an implantable device because it can complicate the task of reliably anchoring the device to adjacent tissue and may raise issues of patient comfort.
Yet another capacitor technology that has been considered for use in implantable defibrillators is the thin polymer film capacitor. Such capacitors employ a thin polymer dielectric film between the metallic capacitor plates, as opposed to the electrolyte dielectric material of the typically employed photoflash aluminum electrolytic capacitor. The plates of the thin polymer film capacitor usually take the form of very thin metal layers that are vapor deposited directly to the dielectric substrate to a thickness of about 150 to 350 angstroms. The result is a so-called metallized polymer film that provides both the dielectric and plate functions of the capacitor. Typically, two layers of metallized polymer film are overlaid and are tightly wound about a mandrel to form a wound cylindrical capacitor. The metallized layers on the two polymer films are offset from opposite respective edges of the films, allowing alternate plates of the spiral-wound structure to be soldered together at opposite ends of the cylindrical capacitor and connected to respective leads. A capacitor wound from metallized polymer film can be constructed with a relatively high energy density because of the efficient use of space permitted by the extremely thin metal plates, and because working voltages well in excess of 700 V can be achieved in a single capacitor. The energy density that can be achieved is limited primarily by the manufacturability of polymer films of arbitrarily small thickness, and by the dielectric properties of the particular polymer film, which dictate the minimum thickness required for a particular design voltage. Energy densities of about one (1) Joule per cubic centimeter are typical for polyester film capacitors, for example. Polyester has a dielectric constant of about 2.5 to 3.0.
An advantageous characteristic of the metallized, thin polymer film capacitor is its ability to self-heal, or clear, minor defects in the dielectric when subjected to an initial clearing voltage greater than its designed working voltage. This characteristic provides a capacitor of high reliability. During clearing, imbedded foreign particles or micro-flaws in the capacitor dielectric lead to localized breakdowns of the film dielectric. The breakdown event results in an arc between the two metallized layers that develops a localized high temperature and pressure. A puncture develops in the polymer film, and the thin metallized plate in the vicinity of the failure site is rapidly vaporized and blown away from the puncture. The evaporation of the electrode around the arc causes it to extinguish, which electrically isolates the two plates on either side of the dielectric film in the vicinity of the puncture. This prevents large-scale damage and catastrophic failure of the capacitor. The clearing process removes an electrode area that is a very small percentage of the entire area of the capacitor plate electrodes, resulting in no significant loss of capacitance. As a general rule, the more flexible and elastic the film material is, and the lower the pressure between the winding layers, the greater the probability that a puncture will self-heal. When inter-layer (radial) pressures are high, the gas pressure associated with the arc increases rapidly, damaging adjacent layers and extinguishing the arc prematurely. This incomplete burning leaves behind a carbon residue that continues to conduct, leading to a thermal runaway that melts many layers of metallized plastic film and generates a catastrophic, high resistance short.
Some polymer films demonstrate better clearing characteristics than others do. In general, polymers that burn well, i.e., that will sustain a flame once ignited, have good clearing properties. Such polymers usually have oxygen in their molecular structure, e.g., polyester, but there are notable exceptions, such as polypropylene.
One promising polymer film for constructing a high energy-density thin film capacitors is polyvinylidene fluoride, or PVDF. This material has a very-high dielectric constant, i.e., k=12, which presents the possibility of constructing a capacitor with very thin films. This would permit more windings within a given capacitor diameter, which increases the plate area within a given cylindrical volume and increases the energy density. Energy densities of about 4 Joules per cubic centimeter are possible with a PVDF dielectric. Also, PVDF exhibits lower leakage than aluminum electrolytic capacitors, with leakage currents on the order of tens of micro-amps rather than hundreds or thousands of microamps. Compared with polyester, however, PVDF has relatively poor selfhealing, or clearing, characteristics.
Evaluations of capacitors constructed using metallized thin films of PVDF have shown electrical degradation at voltages lower than expected, considering the inherent voltage breakdown characteristics of PVDF. For example, two metallized layers of PVDF were cylindrically (spirally) wound on a mandrel having a diameter of about 2 to 3 mm. The layers were wound until the capacitor had a diameter of about 14.5 to 15 mm, with a height of about 50 mm. The PVDF film had a thickness of about 6 microns, and the metallized layers were offset about 2.5 mm from respective opposite ends of the cylindrical construct. In theory, such a capacitor should have withstood at least 2000 V without breakdown, but in fact exhibited voltage breakdown at about 800 V to about 1050 V. Subsequent examination of the failed capacitors revealed many successful clearings of minor defects, as well as some catastrophic failures involving localized voltage breakdown through several layers of dielectric film. The catastrophic failures had not taken place at locations distributed uniformly over the film, but rather had been concentrated at the beginning (near the mandrel) and at the end (on the surface of the capacitor) of the film. It was noted that the failures at the end of the windings were due to shorting between the edges of the two films. The polymer film from which the capacitor had been wound had not been de-metallized at the last few turns. It was also noted that the film windings at the center of the capacitor, i.e., at the beginning of the winding near the mandrel, were very wrinkled. The wrinkling is believed to have been caused by the winding process in which the first few turns resist bending smoothly at the small radius involved. The wrinkling may have resulted in localized areas of high inter-layer pressure in which breakdown events that ordinarily would have terminated in a self-healing, nevertheless cascaded through several layers into catastrophic failure.
It would be desirable to provide improvements in the design of and manufacturing steps for making thin film capacitors to permit the full potential of very thin films of PVDF to be exploited to increase the energy density of the capacitor. These and other advantages are provided by the present invention.